Norsigian and Arnold Peter, a California lawyer, hope that the doubters will be silenced by the findings of an authentication team they put together to study the pictures.

Among evidence they cited on Tuesday was that of two handwriting experts who confirmed that the writing on the envelopes is that of Adams's wife, and a meteorologist who concluded from the cloud formations that at least one of the pictures were taken the same day and time as a famous one by Adams.

"It truly is a missing link of Ansel Adams and history and his career," said David Streets, a Los Angeles art dealer and appraiser who is hosting an exhibition of the photographs.

Norsigian is charging $7,500 for prints from the negatives and Mr Streets estimated that the set could be worth at least $200 million.

Patrick Alt, a photography expert who believes the negatives are authentic, has said the collection showed Adams "trying to discover his voice" and "illuminates a very important part of his evolution as an artist because this is the work that he did in his twenties".

He added: "In almost all of the photographs, the compositions are virtually flawless, truly being made by a photographer of singular vision and talent."